Fair value measurements Fair Value Measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 57	$ 44	$ 41
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	1	(2)	(1)
Net Realized Gains/Losses, Assets	(2)	6	2
Purchased, assets	34	37	24
Sales, assets	(21)	(28)	(22)
Settlements, assets	0	0	0
Balance- End of year, assets	69	57	44
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(3)	(1)	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	1	0	25
Transfers into Level 3, assets	0	5	0
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	6	9	16
Sales, assets	0	0	0
Settlements, assets	(1)	(13)	(41)
Balance- End of year, assets	6	1	0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	11	263	225
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	(252)	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	(2)	6
Net Realized Gains/Losses, Assets	0	1	0
Purchased, assets	0	50	56
Sales, assets	0	0	0
Settlements, assets	(1)	(49)	(24)
Balance- End of year, assets	10	11	263
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	1	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0
Available-for-sale Securities [Member] | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	345	93	74
Transfers into Level 3, assets	11	13	0
Transfers out of Level 3, assets	(24)	(2)	(3)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	13	(12)	3
Net Realized Gains/Losses, Assets	(1)	(3)	0
Purchased, assets	228	334	84
Sales, assets	(70)	(69)	(59)
Settlements, assets	(53)	(9)	(6)
Balance- End of year, assets	449	345	93
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	(1)	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	7
Available-for-sale Securities [Member] | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	1,299	1,037	681
Transfers into Level 3, assets	23	24	231	[1]
Transfers out of Level 3, assets	(38)	(31)	(93)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(2)	(4)	(12)
Net Realized Gains/Losses, Assets	(4)	(5)	0
Purchased, assets	577	672	521	[1]
Sales, assets	(125)	(164)	(111)
Settlements, assets	(279)	(230)	(180)
Balance- End of year, assets	1,451	1,299	1,037
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(2)	(7)	(2)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(8)
Available-for-sale Securities [Member] | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	61	78	45
Transfers into Level 3, assets	0	1	50
Transfers out of Level 3, assets	(16)	(3)	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	19	5	8
Sales, assets	(1)	0	(1)
Settlements, assets	(3)	(20)	(24)
Balance- End of year, assets	60	61	78
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	$ 0	$ 0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	$ 0

[1] Transfers into and Purchases in Level 3 primarily consist of privately-placed fixed income securities.